I need to replace the Acc. Belt Tension Damper on my 87 300 SDL. Looks fairly straight forward. Remove the hex bolt at the top and bolt at the bottom. Is their anything I might be overlooking?

19 mm bolt at bottom first! Before bolt on top or you will destroy tensioner if you are going to loosen the belt.

yal, that's SDL M103 owners can only envy the simplicity of the OM603 belt tensioner.

87, that's about all there is to it. If you can't get a wrench on the lower bolt, you'll have to remove the belt and pivot the tensioner arm for better access to the lower damper bolt.

Sixto
95 S420
87 300SDL

Cool. The M103 and M102 is just plain evil

Let me ask you all this, if the tension spring has gone bad then the belt would be loose when the engine is off, right? The reason I ask is that I hear a knocking sound outside of the normal diesel engine sound. When I look at the belt while the engine is running, the tensioner is moving up and down and the belt looks like it is going to slip off.

I suggest you do more investigation. Remove the belt and have a good look as the spring, tensioner arm and tensioner pulley. Actually, before you remove the belt, look down at the tensioer pulley and check whether it's absoultely parallel with the other pulleys or askew. As bearings wear it skews such that, as viewed from the top, the pulley is closer to the block on the left and closer to the radiator on the right. With the belt out check that the tensioner pulley spins freely with no noise and doesn't wobble. Check that the tensioner arm siwngs freely without clicks or detents and doesn't wobble. If those parts check out then the spring is worn. I certainly wouldn't put a new spring on a worn tensioner arm. If you have to replace the tensioner arm then it's a good measure to get a new spring.

The damper is the least critical component. It makes a frightening noise when the end fittings go bad but it's really only used to hold the belt in place when there is sudden load on the belt system such as when the AC compressor engages. If the belt flops around when the engine is idling I doubt that it's caused by a worn damper.

FWIW the tensioner arm attaches to the timing chain cover through a bolt that takes a 12mm hex bit. I'm able to replace the tensioner arm in my SDL without removing the metal fan. I use a 12mm head bolt with a couple of jam nuts and a socket on a 3/8" breaker bar between fan blades.

Sixto
95 S420
87 300SDL